Motor vehicle bodywork parts are secured to the structure of the vehicle by add-on mounts which hold them in position throughout all stages of manufacture of the vehicle subsequent to them being put into place, and also throughout the lifetime of the vehicle.
The steps in vehicle manufacture subsequent to bodywork parts being put into place include the step of electrophoresis which consists in immersing the entire vehicle, including its bodywork parts, in a bath, and then passing the vehicle through an oven at a temperature lying in the range about 180.degree. C. to 220.degree. C. so as to treat the structure of the vehicle and all of the metal parts it includes against corrosion, and also a painting step which comprises a stage of baking at a temperature lying in the range about 140.degree. C. to 170.degree. C.
Those various operations are designed for bodywork parts made of sheet metal.
However, bodywork parts made of plastics material withstand such treatment less well, particularly the baking treatment for paintwork.
During heating, the plastics material parts expand to a much greater extent than do the structural parts of the vehicle, and as a result a large amount of differential expansion arises between the structure of the vehicle and the bodywork parts that are mounted thereon. As a result the bodywork parts made of plastics material deform and warp or form corrugations that spoil the appearance of the vehicle.
In addition, differential expansion can occur during the lifetime of the vehicle, given that its parts can reach temperatures of close to 80.degree. C., and that also needs to be taken into account in the way bodywork parts are mounted to the structure of a vehicle.
Finally, another problem arises because of the shrinkage to which thermoplastics material parts are subject after a rise in temperature and over time, which phenomenon is known as "post-shrinkage", giving rise to bodywork parts moving relative to the structure of the vehicle, independently of phenomena of differential expansion.
To solve those drawbacks, add-on mounts are used that enable a bodywork part to slide relative to the structure of the vehicle. Such mounts are referred to as "sliding mounts".
French patent application No. 96 04317, published under the number 2 747 091, describes such a sliding mount, which is provided with spring blades for returning it towards a privileged position.
That sliding mount gives rise to various problems which are associated with the presence of said spring blades.
The privileged position of the sliding mount corresponds to an equilibrium position in which the spring blades exert opposite forces of the same amplitude on the mount. Consequently, this position depends on the stiffnesses of the two blades and can vary from one mount to another if the stiffnesses of the blades are not stable in production.
Furthermore, the plastics material used for making the sliding mount is preferably creep-resistant, thereby making it rather brittle and not very flexible, whereas the spring blades need on the contrary to be elastically deformable.
As a result, in order to make the sliding mount, it is necessary to select an intermediate plastics material that provides a compromise between those two contrary requirements, or else the spring blades must be made of an elastic material that is different from the remainder of the sliding mount.